1. Field of the Invention
The present invention relates to a method of and an apparatus for obtaining a tone curve or a tone conversion curve used for executing tone conversion of image data which are obtained by reading an original image.
2. Description of Background Art
In an image processor such as a color scanner, tone conversion is conducted on image data which are obtained by reading an original image by photoelectric scanning thereof. Thereby a recorded image having required tone expression is obtained. For such tone conversion, it is preferred not to employ a fixed tone curve but to employ a tone curve in accordance with the characteristic of the original image to be processed. However, for producing an appropriate tone curve for each original, the structure of the image processor is liable to become complicated to some extent.
Efforts have been made to develop a simple to use and construct technique for obtaining tone conversion or gradation conversion tables in accordance with the characteristics of each original. For example, in the technique disclosed in Japanese Laid-Open Application Laid-Open No. 63-42575 (1988), a required tone curve is obtained by combining a tone curve determined according to a density histogram of the whole image and a reference tone curve prepared in advance, in accordance with an arbitrary combination ratio.
However, for particularly appropriate expression of the tone of a partial region in the original image, the tone curve produced by the aforesaid technique is sometimes inadequate. When the color tone of the partial image region whose tone is desired to be expressed appropriately is widely different from the color tone of the whole image, the tone expression of the partial image region might not meet the requirement, because the tone curve is obtained according to the density histogram of the whole image in the aforesaid technique.